A severe thunderstorm warning was issued for southeastern Hunterdon County, all of Mercer County, southwestern Somerset County, northwestern Middlesex County and southeastern Bucks County in Pennsylvania, effective until 6:15 p.m.

It was posted by the National Weather Service just before 4:30 p.m., when radar indicated a severe thunderstorm near Levittown, Pa. and moving northeast at 20 mph.

"This storm is capable of producing one-inch diameter hail and damaging winds in excess of 60 mph," the weather service said. The storm was predicted to cross the Delaware River into New Jersey around 4:45 and head toward Washington’s Crossing, then on to Princeton and Hopewell.

"This is a dangerous storm,” the agency said. “If you are in its path, move indoors to a sturdy building and stay away from windows.”

"Frequent cloud-to-ground lightning is occurring with this severe thunderstorm,” NWS reported. “When thunder roars, move indoors! Remember ... if you can hear thunder, you are close enough to be struck by lightning.”

A severe thunderstorm watch remains in effect for the region until 9 p.m. today. The watch area includes 16 counties: Hunterdon, Warren, Morris, Sussex, Somerset, Middlesex, Mercer, and Monmouth and in south jersey, Atlantic, Burlington, Camden, Cape May, Cumberland, Gloucester, Ocean and Salem.